In optical communication systems in intermediate-distance and long-distance networks, larger capacity transmission has been developed due to higher speed operation and wavelength multiplexing. In the recent optical communication systems in trunk networks, wavelength multiplexing optical communication is used and a wavelength channel spacing is set. If the interval is 50 GHz within the bandwidth of an optical fiber amplifier, about 100 channels can be used.
In the wavelength multiplexing optical communication, one wavelength (=channel) of light is used as a carrier. In general, a bandwidth of 50 GHz is used in one channel, and a moderate bandwidth is used to prevent occurrence of crosstalk between adjacent channels. For example, non-return-to-zero (NRZ) signals use a bandwidth of about 10 GHz at a transmission speed of 10 Gb/s, which enables use of all channels without interfering with adjacent channels.
Assuming herein that the channel spacing is Δf[Hz] and the transmission speed is B[bit/s], B/Δf[bit/s/Hz] is referred to as “spectral efficiency”. It is generally known that the theoretical limitation of the spectral efficiency in the NRZ is 1[bit/s/sHz] (Patent Literature 1). In the case where the transmission speed is 10 Gb/s and the channel spacing is 50 GHz, the spectral efficiency is 0.2[bit/s/Hz]. Since the optical fiber amplifier has a limited bandwidth, high-density communications capable of improving the spectral efficiency are desirable in terms of the efficiency in optical spectral regions. However, simply increasing the transmission speed to improve the spectral efficiency causes a problem of crosstalk between channels. In this regard, Patent Literature 1 discloses a technique for applying an orthogonal frequency-division multiplexing (hereinafter “OFDM”) method to optical communications.
Another method is proposed in which the OFDM technique is used to decompose data having a high bit rate into signals having a low bit rate by inverse Fourier series using electric signals, and the signals thus obtained are carried in a plurality of orthogonal subcarriers and optically transmitted (Non Patent Literature 1). This solves the problem of crosstalk and improves the spectral efficiency.
As examples of such an optical OFDM technique using the OFDM in optical communications, there are disclosed a method of generating OFDM signals mainly by electric signal processing (Non Patent Literature 1), a method of generating optical OFDM signals by superimposing data signals to be multiplexed using a multi-wavelength light source having a constant frequency spacing (Non Patent Literature 2), and a method of reducing the number of ports that are not available for data communication in the optical OFDM (Patent Literature 2). Along with the popularization of the Internet, for example, the amount of information to be dealt with by devices, such as servers and routers, has been rapidly increasing. Accordingly, the transmission capacity of signals to be exchanged between semiconductor components such as an LSI (Large Scale Integration) constituting these devices is expected to continuously increase rapidly in the future. Under such circumstances, a higher transmission speed and a larger transmission capacity of signals to be transmitted between semiconductor components such as an LSI are key issues.